1. Field of the Invention
The invention relates in general to a control circuit of a laser diode, and more particularly, to an auto laser power control circuit and a switch control circuit.
2. Description of the Related Art
Currently, the laser diode driving circuit in an optical storage drive (for example, CD-ROM drive and the like) uses a switch control circuit in an auto laser power control to activate (enable) or cut off (disable) the driving circuit of the laser diode, that is, to turn on or off the laser diode. FIG. 1 shows a conventional switch control circuit in an auto laser power circuit for a laser diode. The laser diode 141 is photo-coupled to a sensor diode 111. That is, the sensor diode 111 converts the light intensity emitted by the light emitting diode 141 into an electrical signal. A close loop control is performed by the feedback amplifying circuit for power control. Under certain circumstances, the power supply VDD1 of the conventional auto laser power control circuit 303 in a chip and the power supply VDD2 of the laser diode driving circuit 304 on a print circuit board (PCB) are both 5V. When the control signal LDEN is high, the P-type transistor 161 does not conduct. A bias circuit is formed by the power supply VDD5 of the laser diode driving circuit 304. The transistor 171 thus turns on and a current flows through the inductor 131 to enable the laser diode 141 to emit a laser. When the control signal LDEN is low, the transistor 161 conducts. The voltage level at the node Y1 is almost the same as the power supply VDD5 and the bias circuit is not formed. Therefore, the transistor 171 is cut off and no current flows from the collector terminal thereof. The laser diode 141 cannot emit a laser.
As the operation speed in circuit becomes higher, and the power consumption is required to be lower, the required power supply is gradually reduced. Alternately, as the chip fabrication process is minimized, a smaller power supply on a chip is also required. Therefore, the required power supply of the auto laser power control circuit is also reduced. For example, the required power supply of the auto laser power control circuit is reduced from 5V to 3.3V. When the control signal LDEN is high, the whole circuit operates without problem. However, when the LDEN is low, a voltage difference is formed between the voltage level at the node Y1 (3.3V) and the power supply VDD5 (5V) of the laser diode driving circuit 304. Meanwhile, the current supplied to the laser diode 141 is ten or more mA (supposing that the PNP type transistor 171 of the laser diode circuit 304 has a current gain of 250, so that the operation current flowing through the laser diode 141 is 14.4325 mA). The operation current makes the laser diode shut off incompletely such that the laser diode possesses the characteristics of an LED. Thus, the lifetime of the laser diode 141 is shortened due to such incomplete turn-offs.